We have obtained five independent myeloma variants which no longer express L. Restriction analysis of genomic DNA from these five variants indicates: L gene loss in three variants; rearrangement resulting in apparent alteration of the coding region at the 5' end of the L gene and lack of detectable L transcription in one variant; and no gross change in the organization of the L gene in one variant, but incomplete processing (splicing) of the apparently normal nuclear RNA transcript so that L mRNA is not generated in the latter variant. Several murine B-lymphoma cell lines which express immunoglobulin (Ig) Fc receptors, and Ia antigens, but not complement receptors on the cell surface have generated interesting clones which will be useful in analyzing the molecular bases for some of the unique aspects of Ig expression. Some clones secrete IgG2a containing an H chain of 64,000 daltons while IgG2a containing an 70,000 dalton form of the gamma 2a H chain is expressed on the cell surface. Clones from three lines simultaneously express kappa and lambda L chains, whereas myeloma cells or normal lymphoid cells usually express either kappa or lambda (isotype exclusion). An Abelson virus transformed pre-B cell line which expresses only mu H chain is being studied. Fusion of this cloned line to variant mouse myeloma cells which do not express L chain results in hybrids which express an L chain coded for by the pre-B cell genome, i.e., hybridization induces the next step in differentiation of pre-B cells.